Spark plug and method for making same



Dec. 2, 1958 w. A. BYCHINSKY' ET AL 6 SPARK PLUG AND METHOD FOR MAKINGSAME Filed April 15, 1955 2 Sheets-Sheet 2 inventors United StatesPatent 9 hub SPARK PLUG AND METHOD FOR MAKING SAME Wilfred A.Bychinslry, Ann Arbor, and Nlichael Skunda,

Davison, Mich, assignors to General Motors Corporation, Detroit, Mich.,a corporation of Delaware Application April 15, 1955, Serial No. 501,633

8 Claims. (Cl. 313-137) This invention relates to spark plugs and to aprocess for making same. It has particular reference to spark plugs ofthe type having an unusually long tubular insulator, either straight orangular, enclosed in a concentric metal shell, and designed primarilyfor use as igniters in gas turbine engines.

One of the major problems in the production of extra long metal shieldedspark plugs such as are often required for gas turbine installations isthat of insulator warpage. Such warpage results in frequent insulatorbreakage and poor sealing properties. There is, of course, almost alwaysa small amount of warping which occurs in the firing of any insulatorregardless of size; however, in the case of an exceptionally longinsulator, even slight warpage will result in an insulator havingextremities which are so far out of line as to render it defective andperhaps useless. There is no commercially feasible production techniquepresently known for completely eliminating firing warpage. Thus, it hasheretofore been the practice to merely scrap the rather large portion ofinsulators which are too badly warped. Obviously, this raises productioncosts considerably. Also, it results in a final product which may beinferior since even those insulators which are used may be slightlywarped.

It is an object of the present invention to provide a spark plug and aprocess for making same which serve as solutions to the above-outlinedproblem. In other words, it is an object of the invention to provide ametal shielded spark plug of exceptional length which has increasedaccuracy of dimensions, better sealing characteristics and greaterdurability. Another object is the provision of a process formanufacturing exceptionally long spark plugs and more particularly theinsulators for such spark plugs which results in a more accurate andtherefore a better product and which also results inlower productioncosts due to a substantial reduction in the amount of scrap.

It has been found that slight insulator warpage has no ill efiectprovided that such warpage is near the axial center portion such that itdoes not cause the end portions to be too far out of substantialalignment. The chief difficulties which arise because of inaccuratelyshaped or warped insulators are excessive insulator breakage and poorsealing. When an insulator is warped beyond a certain extent its upperextremity may, for example, contact the metal shield of the spark plugand thus the least amount of shock will cause breakage. Such insulatorwarpage may also result in a poor seating of the gaskets between theinsulator and the shell thereby resulting in a defective gas seal.

As stated previously, the warpage which occurs in firing a relativelyshort insulator is small in comparison to that which occurs in themanufacture of a long insulator. Also, the same amount of warpage in ashort insulator does not cause the end portions thereof to be as far outof alignment as in the case of a long one.

The present invention contemplates an extra long metal shielded sparkplug having an insulator which is made in two axial sections joinedtogether by a suitable bonding material. The two sections may be joinedin axial alignment for use in a straight spark plug or may be joined atan angle to each other for use in an elbowshaped plug such as isrequired for certain types of installations. The invention alsocontemplates an improved method for the production of such spark plugsand more particularly for the production of the insulator elements.Various other objects and advantages to be gained by the presentinvention will appear in the following description and in theaccompanying drawings in which:

Figure 1 is a sectional view of a straight spark plug embodying theinvention;

Figure 2 is a side view of the mating portions of the two insulatorsections shown in Fig. 1 prior to joining during the process ofmanufacture;

Figure 3 is an enlarged view of the insulator joint shown in Fig. 1after the bonding operation is completed during the process ofmanufacture;

Figure 4 is a sectional view of an elbow-shaped spark plug embodying theinvention; and

Figure 5 is a side View in partial section of the insulator joint shownin Fig. 4.

Referring now to the drawings and more particularly to Fig. 1, there isshown a metal shielded igniter plug of the type to which this inventionrelates, namely, one having exceptional length and adapted particularlyfor use in gas turbine or jet engine installations. Its chief componentparts are a cylindrical shaped tubular metal shell 4, a tubularinsulator 6 enclosed by the shell, and a center electrode assembly 8within the centerbore 9 of the insulator.

In the embodiment shown, the metal shell 4 consists of three tubularmetal sections ill, 12 and 14 which may be secured together by anyconventional means such as a flange and shoulder arrangement as shown at16 or by threaded engagement as shown at 18 or by welding or brazing.Upper section 14 is for the purpose of providing radio shielding toprevent interference with radio reception and is thus often referred toas the shield. Screw threads are provided at 29 for securing theignition harness to the igniter in the conventional manner. In theparticular embodiment shown, the lower end of the shell is bent inwardlyto form a flange 22 the inner periphery of which serves as the groundelectrode and the upper annular surface of which serves as a seatingsurface for insulator 6.

insulator 6, which consists of two elongated sections.

24 and 2s bonded together end-to-end at 44 as hereinafter morespecifically described, is secured substantially concentrically withinthe shell 4 by means of the external annular shoulder 25 which is seatedand clamped between the lower edge of the shield 14 and the upper edgeof a metal spacer ring 27 the bottom of which engages the internalshoulder in the shell 4 as shown at 16. Metal gaskets 28 and 39 areprovided to assure a good seating and to form a gas-tight seal.

in the particular embodiment shown, the center electrode assemblycomprises a metal spindle 32, the upper end of which is threadedlyengaged with a nut member 33 and embedded in a conductive glass seal 34,and a metal contact plug 36 having a lower portion which is alsoembedded in the conductive glass seal 34, thereby establishingelectrical contact with the spindle 32. Above the contact plug 36, theinsulator centerbore is of enlarged diameter to accommodate the terminalat the end of the ignition lead. The center electrode spindle 32 extendsto the exterior of the flat end surface of the insulator and terminatesin a head 38 which is in spaced Y prevent undermining and 5 excessiveelectrode wear.

By reference to Fig. 1and theab'ove descriptionit will be obvious thatthe insulator element 6 must be formed relatively accurately. Forexample, if there is more than ju'st'a slight amount-of Warpage, thethin-walled upper end of the insulator will contact the metal shield 14thereby'. subjecting said insulator to mechanical shock and breakage;Also, suchcontact between the insulator and the shield 14' may cause amisalignment of the insulator shoulder 25 with the cooperating metalseating portions or ami'salignment of the base of the insulator with theseat ing surface formed by flange 22 thereby causing a defective gasseal.

To assure the desirable accuracy of alignment and thereby prevent theabove-mentioned difficulties, the relativelylong insulator 6 is, inaccordance with the present invention; constructed of two short sections24 and 26 permanently joinedtogether' by a suitable bonding material at44. For the purpose of assuring a good bonded joint, -the sections areprovided with mating sur-' faces. Thus, in the embodiment shown in Fig.1, the lower end of upper insulator section 24 isprovided With acylindrical projection 46 which fits into and is bonded to a matingcylindrical recess in the upper end of lower insulator section 26. Itwill be noted that sections 24 and 26 are of substantially equal lengthand that the joint 44 is thus located substantially in the middle of theinsulator. In this manner each of the upper and lower sections ismaintained as short as possible thereby reducing warpage difiiculties.Also, as pointed out earlier, a slight amount of inaccuracy near theaxial center of the insulator is of no particular disadvantage. Thus, itwill be obvious that by locating the joint near the middle of theinsulator a substantially perfect alignment of the ends of the insulatorcan be assured, any inaccuracy due to warpage of the small insulatorsections manifesting itself adjacent the joint where it will do no harm.

A more complete understanding of the structure and of the advantages tobe derived therefrom will be had from the following description of ourprocess for making same.

To manufacture the insulator member 6, upper and lower sections 24 and26 are first formed by any one of the well-known techniques such, forexample, as by injection or compression molding wherein an organic resinis used as a binder and subsequently burned out during the firingoperation, by the rubber molding technique utilizing paraflin as abinder which is burned out during firing, or by grinding or turning tofinal shape from a cylindrical blank. Because of its superior qualifiesand desirable characteristics, we prefer to use as the insulatormaterial a sintered high alumina composition such, for example, as iscovered by U. S. Patent 2,272,618 Fessler et al., assigned to theassignee of the present invention.

Next, at least those portions of the insulator sections 24 and 26 to bejoined are coated with a thin layer of ceramic bonding material andfired until said material is fully matured. Then, after cooling, atleast the mating portion of one of the sections 24 or 26 is coated witha second layer of the ceramic bonding material after which the twosections are fitted together as shown in Fig. 3, and fired,Whilepressure is applied, to mature the bonding material and'form thejoint 44.

It is to be particularly noted that joint 44 is preferably attained bytwo distinct coating operations. That is, the mating portions of the twoinsulator sections are first individually coated and fired and then asecond application of bonding material is made to at least one of thetwo sections, after which the sections are fitted together and againfired to complete the joint. We have found that this technique of usingtwo distinct glazing operations results in'a' joint which is farsuperior in strength and other desirable--characteristicsto that whichmay be obtained by means of a single application of bonding material tothe parts.

A variety of ceramic bonding materials can be used, ranging from thosewhich mature at relatively low temperatures of 1600 F. to relativelyhigh temperatures of 2500 F. p

The exactbonding material which can best be used will, of course,be-determined by the composition of the-insulators. Preferred materialsfor joining high-alumina type insulators of the type referred to aboveare thelow temperature maturing glazes. Such glazes are well known tothose skilled in" the art andthe following example is given by way ofillustration and not by way of limitation:

Percent Ferro frit #3285 1 63.21 Ferro frit #3505 V 3005 Ball Clay -e.--6.45 Epsom salts .09 Sodium boro phosphate r .18 Fuchsin dye .02

Ferro frit #3285 Percent SiO 51.52 B 0 25.97 :A1203 3.8 FeO sa 1.31 'LiO 2.26 Na O 6.06 K 0 2.51 CaO 4.10

F erro frit #3505 Percent SiO 33.3 PbO 39.4 B 0 19.8 A1 0 4.6 Li O 2.9

The glaze joint should be as thin as possible'rather than heavy in orderto obtain maximum strength. The bonding material should wet the surfacesof the ceramic and should not craze or'crawl and the joint should befree from blebs-and bubbles, this being accomplished by'the applicationof a slight amount of pressure during the firing of the bondingmaterial.

' If the joint is to be in a position where it may undergo heat andelectrical stress at the same time, the bond should be as free of alkalias possible. If the joint is in such a position as to be in a'reducingatmosphere, it is desirable to use lead free bonding materials.

The' followingis a'more detailed descriptionof the" procedure which maybe used in forming the joint in accordance with the invention, thebonding material used being the above-listed glaze.

The mating portions of the insulator sections 24 and 26- are coated withabout from .003 to .004 inch thickness of the low temperature glaze andfired at 1600 'F. on a 15 minute schedule to allow the glaze to mature.After cooling, the mating portion of one of the sections 24 or 26 isagain sprayed or otherwise coated with a second layer of the lowtemperature glaze, the thickness of this layer being sutficient toprovide a snug fit between the two insulator sections whenfittedtogether as shown in- Fig. 3. The two sections 24 and 26'are thenfitted together, projection 46 extending into the mating well providedat the'top. of insulator section 26. Center electrode spi-ndle'32(together with gasket 4%) is then secured into place by means of thethreaded nut 33 which fitsintothe enlarged diameter counterboreStl ofthe top insulatorsection 24 as shown in Fig. 1., threaded member 33being'left rather loose so as to allow'a small amount of movement of thetwo insulator sections with respect to each other. This assembly is thenplaced in a jig indicated at 52 in Fig. 3 so as to obtain a perfectalignment of at least the extremities of the overall insulator. In thislast regard a small amount of clearance should be provided between themating portions of the two insulator sections so that correct dimensionsand alignment can always be attained at this stage of the process. it isalso to be noted that the center electrode spindle 32 assists inaligning the insulators and in some instances it may not be essential touse an aligning jig.

With insulator sections held properly aligned in the jig 52, the entireassembly including the jig is heated to about 1600 F. on a 15 minuteschedule while the insulator sections are being pressed together at thejoint 44 with a total pressure of about 3 to 4 pounds. After cooling,the threaded nut 33 is tightened on the center wire spindle, thuscompleting the insulator assembly.

It is highly advantageous to use a bonding material which matures at arelatively low temperature since this allows the use of jigs constructedfrom material such as metal having relatively low melting temperatures.In those instances where a high-fired glaze is used, it is necessarythat the jig be of ceramic or a similar material having a very highresistance such that it is not destroyed during firing of the bondingmaterial.

As the next step in the process after the insulator is completed, theconductive glass seal material, consisting of a mixture of glass andmetal powder or similar conducting material, is tamped into the enlargeddiameter counterbore 50 of the insulator, contact plug 36 inserted andthe entire assembly fired so as to soften the glass and form the seal.Examples of such conductive glass seals and processes for forming sameare described in U. S. Patents 2,106,578 and 2,248,415, both assigned tothe assignee of the present invention.

To complete the spark plug assembly, gasket 42 is positioned in thespark plug shell 4 together with the insulatorcenter electrode assembly,spacer ring 27, gaskets 28 and 30, after which shield 14 is screwedtightly into place as shown. The final step is a Cico weld operationwhereby the thin shell section 54 is softened by passage of electriccurrent while pressure is applied on the ends of the shell so as totightly clamp the insulator shoulder 25 into place between the matingmetal portions and firmly seat the base of the insulator against theflange 22.

Figure 4 shows an angular type spark plug embodying the invention andcomprises an elongated cylindricalshaped metal shell 60, acylindrical-shaped insulator 62 concentrically enclosed by the shell andformed of two elongated sections 64 and 66 bonded together end to end at68, and a center electrode assembly 70. The shell 60 is formed of twoangularly disposed portions 72 and 74, the latter of whichconcentrically encloses the thin-walled upper insulator section 64 andconstitutes a radio interference shield. Center electrode assembly 70 isquite similar to that shown in Fig. 1 and includes a spindle 76, seals'78 and 80 and contact plug 82. Electrode spindle 76 terminates in ahead portion 84 which extends to the exterior of the insulator in spacedrelation to the lower end of the shell 60 which serves as a groundelectrode.

The insulator 62 is seated and sealed within the shell by means ofgasket 86 which contacts the surfaces of the shell and insulatorshoulder 88 as shown, and by the gasket 96 which is pressed between thetop surface of the insulator and the shell.

As can best be seen in Fig. 5, the mating surfaces of the insulatorsections 64 and 66 consist of the cylindrical surface of section 64which fits into and is bonded to the surface of a cylindrical recess 92extending transversely across the top of section 66. The bondingmaterial for joining the two sections may be the same as that describedwith reference to the embodiment shown in Fig. 1. Also, the method formaking the spark plug and, in particular, the insulator is preferablythe same as that prea viously described, in this instance the jig usedbeing suitable for attaining the proper angularity between the insulatorsections.

The present invention is particularly important and advantageous withrespect to spark plugs such as shown in Fig. 4 since accurate forming ofan angular one-piece insulator is especially diflicult.

By means of the present invention, accurate alignment of at least theend portions of the insulator with the end portions of the shell isassured. Thus, substantially, concentricity is attained, particularly atthe extremities of the insulator where it is most important. Also,because of this accurate alignment, firm and continuous contact of theinsulator shoulder and base against the mating shell seating surfaces isassured, thereby providing a good seal.

It is to be understood that, although the invention has been describedwith specific reference to particular embodiments thereof, it is not tobe so limited since changes and alterations therein may be made whichare within the full intended scope of this invention as defined by theappended claims.

What is claimed is:

l. A spark plug comprising an elongated tubular metal shell providedwith a ground electrode at one end thereof, an elongated tubular ceramicinsulator secured within and enclosed along its entire length by saidshell, said insulator comprising two elongated sections of substantiallythe same ceramic composition, One of said sections having a recess atone end thereof defined by a generally cylindrical wall and the other ofsaid sections having a cylindrical mating surface extending into saidrecess and permanently bonded to said wall to thereby form a jointbetween said sections, said joint providing substantial alignment of theextremities of said insulator with the extremities of said shell, and acenterwire fixedly secured within said insulator sections having an endportion in spaced relation to said ground electrode to form a spark gaptherewith.

2. A spark plug comprising an elongated tubular metal shell having atleast two spacially separated internal seating surfaces and providedwith a ground electrode at one end thereof, an elongated tubular ceramicinsulator enclosed along its entire length by said shell and providedwith at least two external surfaces adapted to cooperate with saidseating surfaces to secure said insulator within said shell and form aseal therewith, said insulator comprising two elongated sections ofsubstantially the same ceramic composition having adjacent end portionspermanently bonded together to provide substantial alignment of theextremities of said insulator with the extremities of said shell anduniform and continuous contact between said external surfaces of theinsulator and the seating surfaces of said shell, and a centerwirefixedly secured within said insulator sections having an end portion inspaced relation to said ground electrode to form a spark gap therewith.

3. A spark plug comprising an angular elongated tubular metal shellhaving at least two spacially separated internal seating surfaces andprovided with a ground electrode at one end thereof, an elongatedtubular ceramic insulator concentrically enclosed along its entirelength by said shell and provided with at least two external surfacesadapted to cooperate with said seating surfaces to secure said insulatorwithin said shell and form a seal therewith, said insulator comprisingtwo elongated sections of substantially the same ceramic compositionpermanently bonded together at an angle with respect to each other toprovide substantial alignment of the extremities of said insulator withthe extremities of said shell and uniform and continuous contact betweensaid surfaces of the insulator and said seating surfaces of the shell,and a centerwire fixedly secured within said insulator sections havingan end portion in spaced relation to said ground electrode to form aspark gap therewith.

4. A spark plug comprising an elongated straight cylindrical shapedtubular metal shell having at least two spacially separated internalseating; surfaces and provided with a ground electrode at one endthereof, an elongated cylindrical shaped tubular .ceramic insulatorconcentrically enclosed along its entire length by said shell andprovided with at least two external surfaces adapted to cooperate withsaid seating surfaces to secure said insulator within said shell andform a seal therewith, said insulator having a lower elongated sectionwith a cylindrical recess at the upper end thereof and an upper sectionwith a bore of enlarged diameter toward the upper end thereof and acylindrical surface at the lower end thereof extending into and bondedwithin said recess to form a straight joint between said sections, saidjoint providing substantial alignment of the extremities of saidinsulator with the extremities of said shell and uniform and continuouscontact between the said external surfaces of the insulator and theseating surfaces of said shell, and a centerwire fixedly secured withinsaid insulator, said centerwire comprising a conductive glass sealwithin said bore and a metal rod having a lower end in spaced relationto said ground electrode to form a spark gap therewith and an upper endin threaded engagement with a nut member positioned in said bore and inelectrical contact with said conductive glass seal.

5. A method for making an elongated tubular spark plug insulatorcomprising the steps of forming two separate elongated ceramic insulatorsections, applying to one end of each of said insulator sections acoating of ceramic bonding material, firing said insulator sections tomature said ceramic bonding material, applying a second coating ofceramic bonding material over said first coating to the end of at leastone of said insulator sections, pressing the coated ends of saidinsulator sections together and firing said sections to mature saidsecond coating of bonding material and form a bonded joint between saidinsulator sections.

6. A process for manufacturing a metal-shielded spark plug of the typecomprising an elongated metal shell having an elongated tubular ceramicinsulator enclosed along its entire length therein, said processincluding the steps of forming an upper insulator section with acylindrical surface at the base thereof, forming a lower insulatorsection with a cylindrical recess at the upper end thereof, applyingceramic bonding material to said cylindrical surface and the walls ofsaid recess, pressing said cylindrical surface into said recess,inserting a centerwire through said insulator sections to form aninsulatorcenterwire assembly, firing said assembly to mature saidbonding material and form a joint, the extremities of said sectionsbeing maintained in position during said firing step for substantialalignment with the extremities of the metal shell and then securing saidassembly concentrically within the metal shell.

7. A method for manufacturing a metal-shielded spark plug of the typecomprising an elongated cylindrical shaped metal shellhaving anelongated cylindrical shaped tubular insulator enclosed along its entirelength therein, said process: including the steps of forming twoseparate cylindrical insulator sections, applying to one end of each ofsaid insulator sections a coating of ceramic bonding material, firingsaid insulator sections to mature said ceramic bonding material,applying a second coating of ceramic bonding material over said firstcoating to the end of at least one of said insulator sections, pressingthe coated ends of said insulator sections together with a pressure ofabout 3 to 4 pounds while firing said sections to mature said secondcoating of bonding material and form one elongated bonded insulator, theextremities of said sections being maintained in position during saidsecond-mentioned firing step for substantial alignment with theextremities of the metal shell, and securing said elongatedinsulator'within said shell so that said insulator is concentricallyenclosed along its entire length thereby.

8. A method for manufacturing a metal-shielded spark plug of the typecomprising an elongated metal shell having an elongated tubular ceramicinsulator enclosed along its entire length therein, said processincluding the steps of forming two separate cylindrical insulatorsections, applying'to one end of each of said insulator sections acoating of ceramic bonding material, pressing the coated ends of saidinsulator sections together while firing said sections to mature saidbonding material and form one elongated bonded insulator, theextremities of said sections being maintained in position during saidfiring step for substantial alignment with the extremities of the metalshell, and securing said elongated insulator within said shell so thatsaid insulator is concentrically enclosed along its entire lengththereby.

References Cited in the file of this patent UNITED STATES PATENTSJohnson Mar. 18, 1924

